While energy consumption in wireless cellular network nodes has been a topic of interest for some time, and has recently taken on a higher priority in the telecommunication community. When analyzing the energy consumption of wireless access networks it becomes clear that in order to reduce the total energy usage it is prudent to focus on the most abundant network nodes, namely the base stations (BS). See for example an undated paper by P{dot over (a)}l Frenger, Peter Moberg, Jens Malmodin, Ylva Jading and Istvan Gódor entitled REDUCING ENERGY CONSUMPTION IN LTE WITH CELL DTX (IEEE Vehicular Technology Conference; Spring 2011, pp. 1-5). Two driving forces behind improving energy efficiency for mobile network operation are a) some network operators are experiencing increased electricity costs for network operation, and b) there is an increased awareness of how energy use relates to green-house gas emissions and global warming. To this end and others there is a new research initiative called EARTH (Energy Aware Radio and neTwork tecHnologies) in which industry and academia have joined to address energy consumption in mobile systems. See for example the EARTH project website (http://www.ict-earth.eu/36.927).
For network energy savings it is prudent to consider how cellular networks are expected to be deployed in the future. One deployment likely to gain wide adoption over time is to use a large number of small cells, which aid in satisfying the increasing data rate requirements in cellular networks; for general background on this see for example the textbook by W. Webb entitled WIRELESS COMMUNICATIONS: THE FUTURE (published by John Wiley & Sons; 2007). The shorter coverage ranges (tens or a few hundreds of meters) of small cells lend themselves well to the use of higher frequency bands that are suited for high data rates. Some companies have proposed that for LTE Release 12 there will be an enhanced Local Area (eLA) small cell as the mainstream scenario. FIG. 1 depicts such a small cell deployment with many small cells densely deployed in a quite high frequency for offloading data (sometimes they are referred to as for providing enhanced services), and a macro cell is deployed for mobility (basic cellular coverage) in a lower frequency.
3GPP TS 36.927, V10.1.0 entitled “Potential solutions for energy saving for E-UTRAN” summarizes four different proposals for energy savings in cellular networks. In a first approach when the coverage cell detects a high load, it uses a proprietary algorithm to decide which hotspot cells should be activated and relies on pre-defined low-load periods' for each neighbor hotspot cell. This appears to be a quite complex algorithm and not able to adapt to the network's fast offloading needs. In a second approach when the coverage cell detects a high load it can request some dormant hotspot cells to switch on their listening capability to perform and report Interference over Thermal (IoT) measurements. IoT is well known in the art, defined in 3GPP TR 36.214 V10.1.0 (2011-09) which is a technical report on potential solutions for energy saving for E-UTRAN. This appears to wake up some cells unnecessarily, limiting its energy saving effectiveness.
When the Coverage cell detects a high load in the third approach, it can request some dormant hotspot cells to transmit a pilot/reference signal for a short time interval which is termed a probing interval. After this interval, all or some hotspot cells will return to the dormant mode while the coverage cell configures its UEs to perform reference signal measurements from the hotspot cells during this interval and report their feedback. This is seen to be similar to the approach defined for mobility purposes in 3GPP TS 36.331. The coverage cell will then determine from the measurement results which hotspot cells should be switched on. This technique also appears to unnecessarily wake up some small cells, and additionally the user equipment (UE) needs to perform measurements at certain intervals which further reduces the overall energy savings.
And finally when the coverage cell detects a high load in the fourth approach, it can use a combination of UE locations, cell locations, and cell radii/transmit powers to decide which hotspot cells should be switched on. There is also a timer value in the activation request message sent from the coverage cell which the selected hotspot cells use to verify at its expiry if the condition required for staying on has been met and if not the hotspot cell will autonomously turn off. This approach does not appear very practical for current cell deployments.